Add gn to DEPS file
Add gn to DEPS file

Pull the gn binary from google source. This is needed inorder to get git cl
format to work correctly on mac. Added other platforms for completness.

R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1826663002 .

DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium